1. Field of the Invention
The present invention relates to an over-current protection device, and more particularly, to an over-current protection device that can provide a uniform and stable conductive strength and reliability.
2. Description of Related Art
As portable electronic devices such as mobile phone, laptop computer, portable video camera and personal digital assistant, etc., are widely used, the importance of the over-current protection device, used to prevent electronic devices from the occurrence of over-current or over-temperature, is increased. Since the positive temperature coefficient (PTC) over-current protection device possesses advantages of reusability being sensitive to temperature variation and, high reliability etc., it is very commonly and widely used in high-density circuit boards and the above-mentioned portable electronic devices.
The PTC over-current protection device uses a positive temperature coefficient conductive material as a current sensor. The resistance of the PTC conductive material is very sensitive to temperature variation, which can be kept extremely low at normal operation so that the circuit can operate normally. However, if an over-current or an over-temperature event occurs, the resistance will simultaneously be increased to a very high resistance state (e.g. above 10,000 ohm.) Therefore, the over-current will be reversely eliminated and the objective to protect the circuit device can be achieved.
FIG. 1 is a schematic diagram of a PTC laminate 10 according to the prior art. The detailed inner structure of the PTC laminate 10 is available in U.S. Pat. No. 6,377,467, entitled xe2x80x9cSURFACE MOUNTABLE OVER-CURRENT PROTECTING DEVICE.xe2x80x9d From the side view, the PTC laminate 10 comprises a PTC material layer 11, an upper electrode foil 13 and a lower electrode foil 14 covering the PTC material layer 11, a first metal layer 15 electrically connecting the upper electrode foil 13, a second metal layer 16 electrically connecting the lower electrode foil 14, a solder mask 18 disposed between the first metal layer 15 and the second metal layer 16, and an insulating layer 17 isolating the upper electrode foil 13 from the second metallic layer 16 and the lower electrode foil 14 from the first metallic layer 15. From the top view, the PTC laminate 10 comprises a plurality of conductive through holes 12 and each conductive through hole 12 is electroplated with conductive material inside. A cutter is used to cut off the conductive through hole 12 along the center to form a half-circular conductive through hole 21, and an packaging process is performed to complete the over-current protection device 20, as shown in FIG. 2.
As the size of the electronic devices shrinks, the size of the traditional over-current protection device also shrinks from 1812 (lengthxc3x97width) and 1210 (lengthxc3x97width) to 1206 and 0805, and even to 0603 and 0402. As the size of the traditional over-current protection device is smaller than 0603, the thickness of the cutter is approximately the same as the diameter of the conductive through hole 12. In this condition, an error on cutting the conductive through hole 12 generally forms an over-current protection device that has a conductive through hole with smaller surface. This will decrease the solderability of the over-current protection device 20 for surface mounting onto a circuit board. Moreover, the material tension and extensibility of the PTC material layer 11 are both larger than those of the metal material under the high voltage condition, which influences the reliability of the PTC over-current protection device 20 on the conductive through holes.
Since the conventional over-current protection device 20 possesses the above-mentioned defects, it is necessary to provide an effective solution for these defects.
The objective of the present invention is to provide an over-current protection device, which can enhance the conductive strength and the reliability.
To this end and to avoid the defects in the prior art, the present invention discloses an over-current protection device, which comprises a positive temperature coefficient material layer, an upper electrode foil, a lower electrode foil, a first metal terminal layer, a second metal terminal layer, and at least one insulating layer. The upper electrode foil is disposed on the upper surface of the positive temperature coefficient material layer, and the lower electrode foil is disposed on the lower surface of the positive temperature coefficient material layer. The first metal terminal layer electrically connects the upper electrode foil with at least one non-full-circular conductive through hole and at least one full-circular conductive through hole, and the second metal terminal layer electrically connects the lower electrode foil with at least one non-full-circular conductive through hole and at least one full-circular conductive through hole. The insulating layer isolates the upper electrode foil from the second metal terminal layer and the lower electrode foil from the first metal terminal layer.
The present invention further discloses an over-current protection device, comprising at least two over-current protection modules, a first metal terminal layer, a second metal terminal layer, and at least one first insulating layer. The at least two over-current protection modules are stacked vertically and are electrically connected in parallel, comprise a positive temperature coefficient material layer, an upper electrode foil and a lower electrode foil. The first metal terminal layer electrically connects the upper electrode foil of the at least two over-current protection modules with at least one non-full-circular conductive through hole and at least one full-circular conductive through hole. The second metal terminal layer electrically connects the lower electrode foil of the at least two over-current protection modules with at least one non-full-circular conductive through hole and at least one full-circular conductive through hole. The insulating layer isolates the second metal terminal layer from the upper electrode foil of the uppermost over-current protection module, the first metal terminal layer from the lower electrode foil of the lowest over-current protection module, and the adjacent over-current protection modules from each other.